Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system (intake passage), a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions and improve fuel economy. An EGR system may include various sensors to measure and/or control the EGR. As one example, the EGR system may include an intake gas constituent sensor, such as an oxygen sensor, which may be employed during non-EGR conditions to determine the oxygen content of fresh intake air. During EGR conditions, the sensor may be used to infer EGR based on a change in oxygen concentration due to addition of EGR as a diluent. One example of such an intake oxygen sensor is shown by Matsubara et al. in U.S. Pat. No. 6,742,379. The EGR system may additionally or optionally include an exhaust gas oxygen sensor coupled to the exhaust manifold for estimating a combustion air-fuel ratio.
As such, due to the location of the oxygen sensor downstream of a charge air cooler in the high pressure air induction system, the sensor may be sensitive to the presence of fuel vapor and other reductants and oxidants such as oil mist. For example, during boosted engine operation, purge air and/or blow-by gases may be received at a compressor inlet location. Hydrocarbons ingested from purge air, the positive crankcase ventilation (PCV), and/or rich EGR can consume oxygen on the sensor catalytic surface and reduce the oxygen concentration detected by the sensor. In some cases, the reductants may also react with the sensing element of the oxygen sensor. The reduction in oxygen at the sensor may be incorrectly interpreted as a diluent when using the change in oxygen to estimate EGR. Thus, the sensor measurements may be confounded by the various sensitivities, the accuracy of the sensor may be reduced, and measurement and/or control of EGR may be degraded.
In one example, the issues described above may be addressed by a method for an engine comprising: disabling EGR flow responsive to an impact of PCV flow hydrocarbons on an output of an intake oxygen sensor increasing above a threshold when purge flow is disabled, the impact of PCV flow hydrocarbons based a difference between the output of the intake oxygen sensor and an output of a DPOV sensor when EGR is flowing. In this way, EGR adjustments based on intake oxygen sensor outputs affected by PCV flow hydrocarbons may be reduced. As a result, accuracy of EGR control may be increased and engine emissions may be maintained at target levels.
For example, during boosted engine operation when EGR is flowing and PCV flow is enabled, hydrocarbons in the PCV flow may cause a decrease in the intake oxygen measured by the intake oxygen sensor. Therefore, when the engine is boosted and an impact of PCV flow hydrocarbons on the output of the intake oxygen sensor is above a threshold when purge is disabled, an engine controller may disable EGR until the impact of PCV flow has decreased back below the threshold. As a result, the controller may not adjust EGR based on an intake oxygen sensor output impacted by increased PCV hydrocarbons in the intake airflow. In one example, the impact of PCV flow hydrocarbons on the output of the intake oxygen sensor may be based on a difference between the intake oxygen sensor output and an output of a DPOV sensor positioned in a low-pressure EGR passage when EGR is flowing. In another example, the impact of PCV flow hydrocarbons on the output of the intake oxygen sensor may be based on a difference between the intake oxygen sensor output and expected (e.g., estimated) blow-by. In yet another example, the impact of PCV flow hydrocarbons may be based on an indication that an estimation for fuel concentration in engine oil is degraded, the indication responsive to an expected output of the intake oxygen sensor differing from an actual output of the intake oxygen sensor by a threshold amount, the expected output of the intake oxygen sensor based on an estimated fuel evaporation rate from the engine oil. Thus, the threshold amount may indicate an increased amount of hydrocarbons in the intake airflow which are impacting the output of the intake oxygen sensor. In this way, the controller may disable EGR flow when the impact of PCV flow on the intake oxygen sensor is above a threshold and may result in degraded EGR flow control.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.